Power consumption for a two stage dry (oil free) screw compressor is significantly reduced if the interstage and the second stage are both decompressed when the compressor is running unloaded. The problem with decompressing both stages, however, is that if the second stage blowdown valve malfunctions, the interstage blowdown valve will decompress the interstage and leave a large differential pressure on the second stage. This large differential pressure will raise the temperature of the second stage, possibly leading to compressor failure.
Previous compressors avoided the above problem by only unloading pressure from the second stage and not from both stages. The disadvantage, however, of unloading pressure only from the second stage when running the compressor unloaded is that the compressor's power consumption is greater than if both stages are unloaded.
Previous valve mechanisms for compressors have not adequately addressed the problem of simultaneously decompressing two isolated stages. U.S. Pat. No. 3,260,444 to Williams discloses valve mechanisms 104 and 110 which are controlled by the same control line 158 and operate in a similar manner. With valve 104, for example, control line 158 can move piston 130 to control whether pipe 106 is in communication with pipe 113 or pipe 102. The disadvantage with using these valves as blowdown valves for a two stage compressor is that if one valve should malfunction, the other valve may continue to function, possibly leading to compressor failure.
What is desired, therefore, is a reliable mechanism for a two stage dry screw compressor to decompress the interstage blowdown valve when the second stage blowdown valve is activated.